Four-lamp driver circuit for fluorescent lamps

ABSTRACT

A two-transistor, high-frequency, non-saturating inverter for operating four fluorescent lamps connected in a series-parallel arrangement. The inverter is supplied either directly from a DC source, or from an AC source via a full-wave bridge rectifier. Transient supression, power-factor correction, and load-balancing networks are provided. Failure or removal of any one lamp will leave two lamps operating in a fail-safe mode.

United States Patent [191 Jensen Aug. 21, 1973 [54] FOUR-LAMP DRIVERCIRCUIT FOR 3,034,015 5/1962 Schultz 3l5/DIG. 7 FLUORESCENT LAMPS1,988,459 1/1935 Pennybacker 3l5/D1G. 5 3,247,466 4/1966 Ma er 331/113 AX [75] inventor: Stephen A. Jensen, North y Hollywood, Calif. P E R L krzmary xammer oy a e A Radian Industries Inc-1 North AssistantExaminerSiegfried 1-1. Grimm Hollywood, Calm Attorney-Roger A. Maars[22] Filed: Oct. 28, 1971 [2]] Appl. No.: 193,449 57 ABSTRACT Atwo-transistor, high-frequency, non-saturating in- [52] gg g f 5 5 4 3verter for operating four fluorescent lamps connected [51] Int Cl Husk3/3'0 H05b 41/29 in a series-parallel arrangement. The inverter is sup-[58] Fieid 315796 97 205 plied either directly from a DC source, or froman AC 315/254 2 5 source via a full-wave bridge rectifier. Transient K il3 supression, power-factor correction, and loadbalancing networks areprovided. Failure or removal of any one lamp will leave two lampsoperating in a fail- [56] References Cited safe mode UNITED STATESPATENTS 3,579,026 5/1971 Paget 331/113 A x 6 Claims, 2 Drawing Figures*1 5 /4 10.40 544 AA/C/A/G Z a 41/0 POM/[P 51cm? 1 A c COP/Q56 r/o/v r0Q| lM/A/G A c 0C {4MB 10 v A/PUT I (OA/VE'RTE? SPEED UP j g (8)? 5A/ETWO/PA/ h I 1 76aA/J/5A/T 6 H lea L 18 SUP/"FSS/OA/ l A/zrwoexCow/14520.44 A l MW 12- A M paw: 74MS/5N7' 7 1 SUfiP/PASS/a/V/MG'Z'WOPA/ EEJ l\ CL 7/24/1/6 o 02. AND 21 I 25 SPEFD-UP 3 MIMI/9% 20 Q1 5 5200a fl/AGPAM a 4 2.4.14?

Patented Aug. 21, 1973 2 Sheets-Shae t .1.

Patented Aug. 21, 1973 3,754,160

2 Sheets-Sheet 2 I BYW M FOUR-LAMP DRIVER CIRCUIT FOR FLUORESCENT LAMPSBACKGROUND OF THE INVENTION As is well known, hot cathodegaseous-discharge lamps, commonly referred to as fluorescent lamps arecharacterized by a relatively high impedance prior to being started orignited, and a substantially lower impedance once ignition isestablished. Thus, a higher voltage is required to ignite the lamp thanis required to maintain its operating state. Various means have beenprovided heretofore which are capable of providing the relatively highignition voltage necessary, and thereafter limit the operating currentonce the lamp is ignited. As a class, these devices are called lampballasts and comprise a wide variety of circuit devices.

Multiple lamp ballasts are available for operation of a plurality oflamps from a common 60 H commercial power supply. These ballasts allowoperation of fluorescent lamps at a current frequency equal to thecommercial main frequency; usually 60 H Such ballasts suffer fromvarious shortcomings, including audible noise, heavy weight, and lowefficiency compared to that available with high frequency lampoperation. Attempts to utilize high frequency static inverters toovercome these shortcomings have met with difficulties concerningswitching losses in power transistors when operating at frequenciesabove the audible range and at the power levels required for theapplication. These losses are aggravated by the fact that most invertersutilize a saturating magnetic core to determine the period ofoscillation of the inverter. At the time of switching, a large peakcurrent flows in the collector of the conducting transistor as a resultof the saturation of the core. The duration of this current is as longas the transistor storage time and is generally a significant period oftime compared to a period of oscillation. This results in excessiveheating of the transistors and generally renders such circuitsimpractical to operate reliably in the environment afforded by a typicalfluorescent light fixture.

The present invention combines a novel and improved solid state inverterwhich drives a plurality of fluorescent lamps and an operating frequencythat is above the audible range. This is accomplished by means of anonsaturating type inverter which avoids the current peaks discussedearlier. By operating at above audio frequencies, the efficiency offluorescent lamps is substantially improved over that achieved at 60 HThis results in less power consumption for a given unit of light outputthan that achieved by conventional ballasts. The non-saturating invertercircuit described herein yields such high efficiency as to allowreliable operation in high temperature environments such as those foundin recessed fluorescent fixture.

BREIF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustratingan embodiment of the invention for driving four fluorescent lamps from acommercial AC power source.

FIG. 2 is a schematic circuit diagram of the system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT There is shown in FIG. 1 a blockdiagram of a fourlamp driver designed to operate directly from aconventional l 15 volt, 60 Hz, AC power source. The input power isapplied across terminals 1 and 2 which energizes the AC to DCconverter 1. In a typical construction, converter 1 may comprise afull-wave rectifier with a suitable smoothing filter. The positiveoutput voltage on line 4, and the negative output voltage on line 5, are

.supplied to a non-saturating, two-transistor inverter comprisingtransistors Q1 and Q2. The inverter is provided with a pair oftransient-suppression networks 6 and 7, and a pair of timing andspeed-up networks 8 and 9. The inverter serves to convert the 160 voltpeak DC from converter 1 to an AC voltage having a nominal frequency of20 KHZ. This arrangement makes the system insensitive to the main powersupply frequency applied to terminals 1 and 2, which may for examplerange from 50 Hz to 1,000 Hz. Optionally, the system may be connecteddirectly to a 130 volt DC power source (without regard to polarity) atterminals 1 and 2.

Transistors Q1 and Q2 function essentially as switches to produce asquare-wave output which is supplied via networks 8 and 9, respectively,to the input winding of autotransformer 11 via lead 10. Initialapplication of power to terminals 1 and 2 produces a DC voltage on lines4 and 5 which establishes a forward bias on the collector-emitterjunctions of both Q1 and Q2. Transistor Q1 will then be biased intoconduction by base current flowing through the timing network 8 into thebase of Q1. This incipient conduction causes Q1 to be regenerativelyswitched into hard" conduction with the supporting base drive obtainedvia .the input winding of autotransformer 11 (lead 10) and the speed-upnetwork 8.

As conduction continues in Q1, the capacitive element of network 8charges and causes the base current at Q1 to decay until Q1 stopsconducting. This action takes place prior to saturation of the core ofautotransformer 11, thereby obviating high peak currents and the lossesresulting from core saturation.

When Q1 stops conducting, the emitter voltage of Q1 will drop and thevoltage to the second winding (via line 20) on the autotransformer willbecome zero. Conduction will then commence in Q2 until this half of thecycle is terminated by the action of network 9. All timing is dependentupon the properties of the networks 8 and 9, rather than the propertiesof the autotransformer. The resulting square-wave applied to theautotransformer produces an output voltage which is four times the peakAC line voltage at terminals 1 and 2. If, for example, the input supplyis 160 volts peak-to-peak,

' then the high voltage available to the lamp circuit is 640 voltspeak-to-peak. This high voltage is then impressed across lamps 12-15,which are connected in a series-parallel arrangement (as will bedescribed more fully hereinafter The functions of the networks 8 and 9are severalfold. Their first purpose is to supply base current towhichever transistor (Q1 or Q2) is then in conduction, and to cause thebase current thereto to diminish to a value below which conductioncannot be sustained in the initially conducting one of the transistors.Additionally, each of these networks (8 and 9) functions to stronglyreverse bias its associated transistor immedi ately upon cessation ofconduction thereof, so asto positively define the cut-off point and toinsure repeatability. This further insures that no collector current canflow in the associated transistor once its conduction has beenterminated. ln this way, the possibility of current flowing in bothtransistors simultaneously, is eliminated.

Unlike conventional saturating-core type of inverters, this circuitprecludes large peak currents flowing through the transistors. Theresult is an increase in efficiency and a reduction in overall heatrise:

Transient suppression networks 6 and 7, previously referred to, serve toblock any spikes or spurious voltage transients which might be reflectedfrom the autotransformer 11 during the ignition of the lamps 12-15.Also, these networks (6 and 7) protect the inverter transistors under ano-load condition such as would occur if all of the lamps were to beremoved from the circuit while the power is on. This condition producesa reflection from the output having large voltage transients which wouldotherwise damage the transistors by raising the collector-to-emittervoltage (in the cut-off condition) beyond their rated value. Suchtransients are absorbed by networks 6 and 7 and are passive duringnormal operation of the circuit.

Autotransformer 11 has a single continuous winding with a plurality oftaps, the individual functions of which will be explained in detail inconnection with the description of the schematic diagram of FIG. 2. Ingeneral, however, these taps provide base drive to transis-v tors Q1 andQ2, and base current to networks 8 and 9. Additionally, theaforementioned 640 volt peak-topeak square wave, from which the lampoperating power is derived, is obtained from autotransformer 1 1.

The circuit is designed to utilize so-called rapid start lamps (12-15).This type of lamp is provided with electrodes which are continouslyheated during lamp operation. The hot electrodes (viz., cathode) permitssomewhat greater lamp current to be utilized and produces lower overalllighting costs in addition to the convenience of a rapid-start feature.The autotransformer 11 is provided with winding taps that continuouslyprovide the required voltage and current for heating of heaters 16-19and 21-24. When the system is energized, these winding taps cause theelectrodes to be quickly heated, releasing enough electrons within thefluorescent lamp for an arc to be established in response to theapplication of the high voltage from the high-voltage taps.

The voltage from the high-voltage taps is impressed across theseries-parallel four-lamp load through the load-balancing network 25,which serves to uniformly divide the current between the lamps 12-15,and to limit the maximum current to conform to the specifications (viz.,manufacturer's ratings) of the lamps. Since the light output is directlyproportional to the current flowing through each lamp, equal division ofthe available current among the four lamps will assure uniform operatingbrilliance. Further, the network 25 optimizes the power factor bylimiting the peak current demand of the lamp load to a point in theoperating cycle other than the time at which switching of thetransistors Q1 and Q2 occurs. This results in a substantial improvementover the capacitive ballast technique of the prior art wherein the peakof the current load coincides with the point in time of the operatingcycle at which switching takes place. When the transistors switch underconditions of high load current, they necessarily dissipate more powerthan they would if they were switching under low load currents.

Incorporation of the network 25 into the novel circuit of the presentinvention causes the current into the load to be very near zero at themoment when there is a switching transition between transistors Q1 andQ2. The peak current load occurs approximately onequarter of the wayinto the corresponding half-cycle of the inverter operation.

There is shown in FIG. 2 a more detailed diagram of a preferredembodiment of the invention. As in the previous description of theapparatus of FIG. 1, the operating power is applied to input terminals 1and 2. The applied alternating current is rectified via the full-wavebridge comprising diodes 26-29. Filter capacitor 31 (C1) smooths the DCwhich comprises the operating potential for the inverter.

In order to follow a typical cycle of operation, assume as the initialcondition that transistor Q1 is biased into conduction by the basecurrent flowing through resistor R6 into the base of Q1. Additional basedrive is supplied from winding W1 of autotransformer 1 1 via capacitorC2 and resistor R1 and conduction proceeds regeneratively to saturationof transistor Q1. This action will cause capacitor C2 to charge to thelevel of the voltage drop across resistor R1. The increasing voltageacross capacitor C2 will cause the base current of transistor Q] todecay towards 0.

Some of the base current is bled off through diode CR2 and seriesresistor R2, thereby causing the base emitter voltage of transistor Q1to sharply pass through 0 volts. This action assures a well-definedcut-off of transistor Q1.

Diode CR1 is connected between the emitter and the base of transistorQ1. During each cycle when the transistor is cut off, it limits themaximum emitter base voltage to the threshold voltage of the diode,slightly less than 1.0 volt. Otherwise, the full driving voltage fromthe feed back coil would appear across that emitter base and could punchthrough. During the conduction of the transistor, the diode is reversedbiased or cut off.

since polarity from base to emitter has been reversed in order to permitconduction of the transistor.

The above-described regenerative-conduction to cutoff cycle oftransistor Q1 occurs before saturation of the core of autotransformer 11can take place. When transistor Ql stops conducting, the emitter voltagewill drop as will the voltage across winding W3. Similarly, the voltageacross winding W6 will also go to O. inasmuch as winding W6 had beenproviding a cutoff bias for transistor Q2 during the initial portion ofthe operating cycle, its termination will permit the network comprisingcapacitor C3 and resistor R4 to forward bias the base-emitter junctionof transistor Q2. lncipient conduction of transistor Q2 will establish apositivegoing voltage across winding W4, and hence winding W6 also.

This action will sustain a regenerative half-cycle in the same manner asdescribed previously in connection with transistor Q1. Specifically, thebuild-up of a charge on capacitor C3 will ultimately reach a level whereit will cause reverse biasing of the base of transistor Q2.

Diode CR3 and CR4, and resistor R3 perform similar functions torespective counterpart components CR2 and CR1, and R2 previouslydescribed.

A new cycle of operation commences and transistors Q1 and Q2 function asalternately conducting switches applying the supply voltage fromrectifier 3 to windings W1 and W6. The square-wave train produced,generates four times the peak AC line voltage across the overall winding(W1 through W6) of autotransformer 11. This total voltage is connectedacross the seriesconnected pairs of lamps 13,15 and 12,14.

The previously described transient-suppression networks comprise diodesCR5 and CR6, capacitors C6 and C7, C8 and resistor R5.

Capacitor C4 is connected in series between one terminal ofautotransformer 11 (at winding W1) and current transformer TI. Thiscapacitor (C4) functions as a current limiter for the total currentflowing to all four lamps 12-15, by virtue of its capacitive reactance.Transformer T1 has a 1:1 winding ratio and is connected so that the twowindings are bucking. Thus, if one pair of lamps ignites prior to theremaining pair, the resulting current flow through one winding of thetransformer T1 will generate a voltage in the other winding which isadditive to the voltage at the junction of capacitor C4 and thetranformer T1. This increased voltage will insure the ignition of theother pair of lamps.

A second function of transformer T1 is to provide an inductive-reactancecomponent to the lamp load. This is obtained by using the leakagereactance which appears as a series element in the lamp current path. Inthe time domain, this arrangement results in the inverter being lightlyloaded during the actual switching portion of the operating cycle,thereby minimizing switching losses. The current peak, which wouldotherwise occur at the instant of switching, is delayed to a point intime which is about 25 percent into the half cycle. This assures thatthe transistors have ample time to become saturated before the heavycurrent is conducted through them. Also, as has been mentionedpreviously, the combination of capacitor C4 and transformer T1 providesa network which yields a high frequency power-factor correction.

As can be seen, lamps l3 and 15 are connected in series between thehigh-voltage terminals 32 and 33 of autotransformer 11 by reason oftheir having their heaters 19 and 23 connected together. Similarly,lamps 12 and 14 are connected in series, via heaters 17 and 21, andplaced across high-voltage terminals 32 and 33. Of course, it should benoted that each of these series paths includes a respective portion ofthe loadbalancing network (C4, T1). The two series-connected pairs oflamps (13,15 and 12,14) are connected in parallel and each set isprovided with corresponding lowvoltage windings on autotransformer 11for energization of the respective lamp heaters. For example, winding 35powers heater 18 of lamp 13, winding 36 powers parallel-connectedheaters 19 and 23, and winding 37 powers heater 24 of lamp 15 and heater22 of lamp l4.

Capacitor C5 permits two lamps to operate in the absence of theremaining two lamps by partially bypassing the open secondary reactanceof transformer T1, which would otherwise prevent any lamp from operatingif one or more of the other lamps were removed from the fixture orbecame inoperative.

From the foregoing it is seen that there is provided by the presentinvention a novel and improved four-lamp driver circuit having very highoperating efiiciency and will continue to function in a fail-safe modenotwithstanding the removal from service of one or more of the fourlamps.

Modifications of the preferred embodiment described will be apparent tothose versed in the art, without the exercise of invention. Thus, it isintended that the invention be limited only by the appended claims.

What is claimed is:

1. Apparatus for energizing a plurality of hot-cathode gaseous-dischargelamps, comprising:

rectifier means having a pair of input terminals for receiving operatingpotential for said apparatus, and having a pair of direct-current outputleads;

means connected to said output leads for changing the direct-currenttherefrom to an altemating' current having a frequency which issubstantially higher than the frequency of said Operating potentialapplied to said input terminals;

a first pair of series-connected lamps;

a second pair of series-connected lamps;

means connecting one terminal of each of said pairs of lamps in commonto one alternating-current output terminal of said current changingmeans; current dividing means having an input connected to the remainingalternating-current output terminal of said current changing means andhaving a first output connected to the other terminal of said first pairof series-connected lamps, and a second output connected to the otherterminal of said second pair of series-connected lamps, for dividing thecurrent from said current changing means equally between the two pairsof series-connected lamps; said current changing means comprises: asaturating two-transistor inverter; said inverter includes: anautotransformer having a continuous multi-tap winding, one end of saidwinding corresponding to said one alternating-current output terminaland the other end of said winding corresponding to said remainingalternating-current output terminal; and

first and second resistance-capacitance coupled timing network means,each connected between a corresponding one of said two transistors andsaid autotransformer, for cyclically transferring conduction betweensaid two transistors at a rate which is independent of variations in themagnetic circuit of said autotransformer.

2. Apparatus as defined in claim 1 including:

first and second transient suppression network means, each connected toa corresponding one of said two transistors, for preventing spurioustransient voltages from being reflected from said autotransformer totheir respective transistors.

3. Apparatus as defined in claim 1 wherein each of said lamps includes:

a pair of spaced-apart heaters; and, said autotransformer includes aplurality of low-voltage windings, inductively coupled to saidcontinuous multi-tap winding, each of which is operatively connected toa corresponding one of said heaters.

4. Apparatus for energizing a plurality of hot-cathode gaseous-dischargelamps, comprising:

rectifier means having a pair of input terminals for receiving operatingpotential for said apparatus, and having a pair of direct-current outputleads;

means connected to said output leads for changing the direct-currenttherefrom to an alternating current having a frequency which issubstantially higher than the frequency of said operating potentialapplied to said input terminals;

a first pair of series-connected lamps;

a second pair of series-connected lamps;

means connecting one terminal of each of said pairs of lamps in commonto one alternating-current output terminal of said current changingmeans;

current dividing means having an input connected to the remainingalternating-current output terminal of said current changing means andhaving a first output connected to the other terminal of said first pairof series-connected lamps, and a second output connected to the otherterminal of said second pair of series--connected lamps, for dividingthe current from said current changing means equally between the twopairs of series-connected lamps;

said current dividing means comprises:

first and second windings having a 1:1 ratio and inductively coupled inopposition to a common elec tromagnetic circuit, one end of each of saidwindings being connected in common to said current changing means andthe remaining ends of said windings being connected to respective pairsof said series-connected lamps.

5. Apparatus as defined in claim 4 wherein said rectifier meanscomprises: .i

a full-wave diode bridge rectifier, thereby permitting bothalternating-current and unpolarized directcurrent to properly functionas said operating potential.

6. Apparatus as defined in claim 4 including:

capacitive reactance means connected across said remaining ends of saidwindings of said current dividing means.

1. Apparatus for energizing a plurality of hot-cathode gaseous-dischargelamps, comprising: rectifier means having a pair of input terminals forreceiving operating potential for said apparatus, and having a pair ofdirect-current output leads; means connected to said output leads forchanging the directcurrent therefrom to an alternating current having afrequency which is substantially higher than the frequency of saidoperating potential applied to said input terminals; a first pair ofseries-connected lamps; a second pair of series-connected lamps; meansconnecting one terminal of each of said pairs of lamps in common to onealternating-current output terminal of said current changing means;current dividing means having an input connected to the remainingalternating-current output terminal of said current changing means andhaving a first output connected to the other terminal of said first pairof series-connected lamps, and a second output connected to the otherterminal of said second pair of series-connected lamps, for dividing thecurrent from said current changing means equally between the two pairsof series-connected lamps; said current changing means comprises: asaturating two-transistor inverter; said inverter includes: anautotransformer having a continuous multi-tap winding, one end of saidwinding corresponding to said one alternatingcurrent output terminal andthe Other end of said winding corresponding to said remainingalternating-current output terminal; and first and secondresistance-capacitance coupled timing network means, each connectedbetween a corresponding one of said two transistors and saidautotransformer, for cyclically transferring conduction between said twotransistors at a rate which is independent of variations in the magneticcircuit of said autotransformer.
 2. Apparatus as defined in claim 1including: first and second transient suppression network means, eachconnected to a corresponding one of said two transistors, for preventingspurious transient voltages from being reflected from saidautotransformer to their respective transistors.
 3. Apparatus as definedin claim 1 wherein each of said lamps includes: a pair of spaced-apartheaters; and, said autotransformer includes a plurality of low-voltagewindings, inductively coupled to said continuous multi-tap winding, eachof which is operatively connected to a corresponding one of saidheaters.
 4. Apparatus for energizing a plurality of hot-cathodegaseous-discharge lamps, comprising: rectifier means having a pair ofinput terminals for receiving operating potential for said apparatus,and having a pair of direct-current output leads; means connected tosaid output leads for changing the direct-current therefrom to analternating current having a frequency which is substantially higherthan the frequency of said operating potential applied to said inputterminals; a first pair of series-connected lamps; a second pair ofseries-connected lamps; means connecting one terminal of each of saidpairs of lamps in common to one alternating-current output terminal ofsaid current changing means; current dividing means having an inputconnected to the remaining alternating-current output terminal of saidcurrent changing means and having a first output connected to the otherterminal of said first pair of series-connected lamps, and a secondoutput connected to the other terminal of said second pair ofseries--connected lamps, for dividing the current from said currentchanging means equally between the two pairs of series-connected lamps;said current dividing means comprises: first and second windings havinga 1:1 ratio and inductively coupled in opposition to a commonelectromagnetic circuit, one end of each of said windings beingconnected in common to said current changing means and the remainingends of said windings being connected to respective pairs of saidseries-connected lamps.
 5. Apparatus as defined in claim 4 wherein saidrectifier means comprises: a full-wave diode bridge rectifier, therebypermitting both alternating-current and unpolarized direct-current toproperly function as said operating potential.
 6. Apparatus as definedin claim 4 including: capacitive reactance means connected across saidremaining ends of said windings of said current dividing means.